Epithelial - Mesenchymal interactions

Recombination studies
1. NCC + 1st arch (jaw) epithelium = Tooth
2. NCC + other epithelium = No tooth
  1. Therefore we can tell that the jaw epithelium is committing the NCC to become teeth
    1. However, after the oral epithelium commits the NCC, the surrounding ectomesenchyme assumes the dominant role and induces tooth development (controls the epithelial outgrowth)
      1. Overall...
      2. Ectomesenchymal influence
        Induces development of enamel organ (and corresponding 4 cell layers during CAP stage)
        In bell stage, EO takes over.
        Recipricol induction (see below)
        Molar epithelium + Incisal mesenchyme = Incisor tooth
        Incisor epithelium + Molar mesenchyme = Molar tooth
        Therefore can distinguish that mesenchyme is controlling the formation/shape of the tooth
4. Essentially in chronologica order - oral epithelium controller -> Ectomesenchyme controller -> Enamel organ controller (recipricol induction)
Two theories
1. Field theory - in the bud stage, local factors "tell" the mesenchyme to form a particular shape tooth
2. Code theory - Mesenchyme is pre-programmed to become particular shape tooth prior to the bud stage
E-M interactions are not dependent upon cell-cell contact. Chemical messengers such as growth factors like TGF-b, BMPs can diffuse through a mediator filter (DBM) and exert their effect on the cell.

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